DES MOINES, Iowa — Republican Mitt Romney opened a broad and newly abrasive assault on rival Newt Gingrich on Thursday, dispatching surrogates and staff to cast him as unworthy of the GOP nomination and unfit to be president.

Romney is aiming to undermine his rising rival on both personal and professional fronts ahead of the 2012 campaign's opening contest Jan. 3 in Iowa — a reversal by the one-time front-runner who had previously all but ignored his Republican foes.

"He's not a reliable and trusted conservative leader because he's not a reliable or trustworthy leader," former Missouri Sen. Jim Talent, a Romney supporter, said of Gingrich, offering a preview of the attacks Romney's team promised would continue in the next few weeks.

Romney allies also were giving him a boost, announcing a $3.1 million TV ad campaign in Iowa that is expected to include hard-hitting commercials against Gingrich.

Campaigning in South Carolina, Gingrich brushed off the verbal assault and insisted his campaign would not change its above-the-fray approach regarding fellow Republican contenders.

"We're focused on remaining positive," he said.

Gingrich's reluctance to engage may be out of necessity. He dramatically lags Romney in organizational firepower as he tries to rebuild his campaign after an early summer implosion that left it deep in debt.

The onslaught of criticism from Romney and his allies, after months of focusing solely on President Barack Obama and little on his GOP foes, comes as the race has developed into a two-person contest. Gingrich's quick rise in national and early-state polls threatens Romney's claim as the likeliest Republican to be chosen to challenge Obama next fall.

If he's successful in tearing down his main rival, there's no guarantee that in a multi-candidate field he'll end up benefiting from a possible Gingrich fall. At the same time, congenial Iowa voters generally don't look kindly on candidates who engage in negative politics, and they could end up punishing Romney in a state that already presents hurdles for him. Plus, questioning Gingrich's adherence to conservatism could draw attention to Romney's own liabilities as having switched positions on key social issues including abortion and gay marriage.

The candidate, himself, personally stayed out of the fray Thursday, raising money in private in Virginia.

But he's all but certain to weigh in with a sharp critique of Gingrich when he campaigns Friday in Iowa. His efforts there seemingly have boosted the importance of a state where Romney had worked to downplay expectations. Romney also is expected to clash with Gingrich during a nationally televised debate in Des Moines Saturday night.

TV ads against Gingrich are all but assured — either paid for by Romney's campaign or by the Restore Our Future political action committee, which is made up of staffers from Romney's failed 2008 presidential bid.

The group announced Monday that it was running a 30-second ad in Iowa that emphasizes Romney's private-sector experiences while castigating Obama's community organizing and academic background. A spot blistering Gingrich was in the works.

Texas Gov. Rick Perry, struggling to spark enthusiasm anew in his campaign, also unveiled a television ad planned for airing in Iowa assailing both Romney and Gingrich for their past support for an individual health care mandate, a provision at the heart of GOP opposition to the 2010 law Obama signed. It's the first attack ad by Perry, who has spent more than $2 million on advertising in Iowa to little avail.

But Romney was the one dictating the direction of the race Thursday.

For now at least, he left the criticism to Talent and former New Hampshire Gov. John Sununu, who held a conference call with reporters as the day began to dissect Gingrich's statements about Republican Rep. Paul Ryan's budget proposal this year.

Earlier this year, Gingrich's critique of Ryan's plan as "right-wing social engineering" and "imposing radical change" was roundly criticized by Republicans. And Sununu said Gingrich's response illustrates a habit of off-the-cuff speaking and thinking that raises a larger question of fitness to lead

"What he did to Paul Ryan is a perfect example of irrational behavior that you do not want in the commander in chief," said Sununu, a prominent Republican in Romney's must-win state of New Hampshire who served as chief of staff for President George H. W. Bush.

Added Talent: "Speaker Gingrich says interesting and insightful things. He also says outrageous things that come from nowhere, and he has a tendency to say them at a time when they most undermine the conservative agenda."

Meanwhile, Romney's team rolled out a series of emailed memos casting Gingrich as a turncoat Republican on Ryan's plan, which is popular with conservatives. One was titled: "With friends like Newt, who needs the left?"

And Romney's campaign also distributed talking points to allies on Capitol Hill in Washington that indicated the campaign plans to swipe at Gingrich's economic record by arguing that he doesn't have a background in the "real economy." Among the points: "Gingrich creates theories, Mitt creates jobs" and "Gingrich has spent a lifetime operating in theory, while Mitt has succeeded in practice."

Over the past few days, Romney has foreshadowed the attacks by drawing not-so-subtle distinctions with Gingrich, particularly on personal issues.

He started running a TV ad on Wednesday that promoted Romney's stable family life — and stoked questions about Gingrich's motivation to run and temperament to lead. Romney's allies did the same, with Talent and Sununu describing Gingrich, with whom both men worked over the years, as untrustworthy and self-centered.

They also warned that Gingrich's long history as an outspoken antagonist would play into the Obama campaign's hands, were he to be the Republican nominee. Talent, who served under Gingrich in the House in the 1990s, said he was not referring to the House ethics scandal that drove him from the speakership nor his two divorces and extramarital affair.

"If the nominee is Newt Gingrich, then the election is going to be about the Republican nominee, which is exactly what the Democrats want," Talent said.

The campaign also signaled that it won't overtly attack Gingrich's personal problems but will draw contrasts with his rocky personal life nonetheless by pointing out Romney's stable family life — especially in Iowa.

Ann Romney will be in the spotlight in Iowa in the coming days, accompanying her husband during the Friday stop in Cedar Rapids and then hosting an afternoon reception in West Des Moines. She'll also attend an event at Romney's Des Moines headquarters on Saturday ahead of the debate.

Subtlety on the personal front may be a smart strategy for Romney.

"He's got high negatives anyway. If he goes on the attack on personal issues, in Iowa, his negatives go up further," said Doug Gross, who was Romney's Iowa campaign chairman four years ago. "It polarizes him further."

Still, he added, Romney's effort to portray himself as more conservative than Gingrich on budget issues could hurt the former Massachusetts governor among the moderates who could help him beat expectations in the Jan. 3 caucuses. And, Gross said, Romney's revival of the Ryan plan also may annoy some House Republicans who supported it last spring but have been trying all year to soothe worried constituents about proposed changes to the prized health care safety net for older Americans in light of Democratic attacks.

Still, Romney's attack on Gingrich's ability to be president may resonate with Iowans.

A recent Washington Post-ABC News Poll in Iowa shows 43 percent of likely caucusgoers said Gingrich "has the best experience to be president," compared to 16 percent for Romney. They also trusted Gingrich more than Romney by big margins on handling the economy and immigration. Still, Romney is not viewed all that strong on these factors, so he's leaving himself vulnerable to attack, should Gingrich drop his positive pledge.

Less than a month before the Iowa caucuses, Gingrich is running ahead of Romney in polls in Iowa.

Romney now faces a dilemma: try to stop Gingrich with a full-throttle campaign in Iowa and risk falling short ahead of must-win New Hampshire or tentatively engage Gingrich in Iowa and risk allowing him to gain momentum heading into the first-in-the-nation primary Jan. 10.